The present invention relates to an apparatus for injecting molten plastic material into a molding machine and in particular, to an improved bushing arrangement to be incorporated into the injection apparatus.
FIG. 1 illustrates a manifold bushing construction 30 for an injection molding apparatus having a conventional thermal expansion sealing nozzle 32. This type of injection molding apparatus and manifold bushing construction is illustrated in detail in U.S. Pat. No. 4,173,448 to Rees et al.
Subsequent to the development of the thermal expansion sealing nozzle, the spring sealing nozzle was developed. FIG. 2 illustrates a molding system with such a spring sealing nozzle. The substitution of the spring sealing nozzle for the thermal expansion sealing nozzle brought one major problem along with it--namely, a leakage problem. The cause of this problem can be traced to the fact that it is very difficult to manufacture manifold bushing spigots with a length that exactly matches the depth of the corresponding recess in the manifold. Consequently a very small gap may exist at the interfaces 10 and 12. Furthermore, the diameter of the spigot 14 has to be slightly smaller than the bore in between the interfaces 10 and 12. When this happens, the plastic, which is subjected to injection pressure, exerts a large separating force at the interfaces 10 and 12 by virtue of the comparatively large projected areas on which it can act. This separating force can overcome the spring force trying to maintain the seal at the interface and leakage can occur.
Other prior art arrangements of manifold bushing and nozzle assemblies face still other problems. For example, U.S. Pat. No. 5,022,846 to Schmidt shows a nozzle and a manifold bushing screwed together through the manifold with bolts. This construction means that when the manifold thermally expands laterally, the nozzle must travel laterally as well or deflect, thereby causing premature wear and the possibility of leakage.
U.S. Pat. No. 4,043,726 to Tsunemoto shows a construction having a spring loaded nozzle assembly with an adjustable set screw. The valve stem and the valve action are within the nozzle body. Injection pressure opens the valve and spring pressure closes it. Lateral thermal expansion takes place along the connecting pipe as it slides inside the nozzle body. The disadvantage of this method is that a double acting air operated valve stem is difficult to incorporate in this design. The publication "The Heat-Lock distribution system" by Nil Helldin AB of Sweden illustrates a similar system.
Finally, it is known in the art that it is often necessary to block hot runner channels in a manifold. This is because the manifold 20 has several drilled channels, the ends of which have to be plugged in order to obtain a desired flow path. FIG. 3 illustrates a plugging system used in connection with one such manifold to ensure a safe, leak free manifold. In this system, the end of a hole 22 in the manifold is tapered and threaded. Then a tapered plug 24 is fitted and inserted and held in place with a threaded set screw 26. The inside of the plug 24 is machined in place to match the contour of the channel. On a multicavity manifold, this procedure is both time consuming and expensive. In order to clean out the channels, each plug has to be removed and new ones installed and machined in place.
U.S. Pat. No. 3,849,048 to Bielfeldt shows a hydraulically actuated piston housing that takes up the cold clearance to prevent leakage. This piston acts like a spring. Inside the housing is a second hydraulic piston which drives the valve stem. The nozzle body is threaded into the manifold insert and therefore thermally expands laterally when the manifold expands. The close proximity of flammable hydraulic oil to the heated manifold means that there is a great risk of fire with this design after the seals have worn.
U.S. Pat. No. 3,716,318 to Erik shows a combined nozzle/manifold bushing piece which is inserted through the manifold from the underside and is retained by a threaded piston housing. This construction is also disadvantageous in that the nozzle assembly must travel laterally with the manifold as it thermally expands.
U.S. Pat. No.3,252,184 to Ninneman shows a manifold bushing piece inserted through the manifold and butted against the spigotted end of the nozzle body. Because the nozzle body is spigotted to the manifold, it must travel laterally when the manifold thermally expands.
U.S. Pat. No. 3,023,458 to Seymour illustrates a one piece manifold bushing and nozzle body inserted through the manifold. The valve stem is closed with a spring and opened via injection pressure. The nozzle end of the bushing appears to be located in a recess in the mold cavity plate and clearly cannot accommodate lateral thermal expansion of the manifold plate with respect to the cavity plate. In effect, bending occurs which would tend to cause the valve stem to bind.
Accordingly, it is an object of the present invention to provide an improved apparatus for injecting plastic material wherein the possibility of having leakages of the plastic material is significantly reduced.
It is a further object of the present invention to provide an apparatus as above wherein lateral expansion of a manifold is not transmitted to a nozzle assembly through which said plastic material flows.
It is still a further object of the present invention to provide an apparatus as above wherein the need to plug holes in a manifold is performed in a simpler and less expensive fashion.
It is yet a further object of the present invention to provide an improved bushing arrangement which has utility in valve gated applications and in non-valve gated applications.
Still other objects and advantages of the present invention will become more apparent from the following description and drawings in which like reference numerals depict like elements.